The use of implantable stents in the vasculature and other body conduits has become commonplace since first proposed by Dotter in the 1960s. These devices are required to have a small, compacted diameter for insertion into an intended body conduit and transport, typically via a catheter, to a desired site for deployment, at which site they are expanded to a larger diameter as necessary to fit interferably with luminal surface of the body conduit. Balloon expandable stents are expanded by plastically deforming the device with an inflatable balloon on which the expandable stent was previously mounted in the compacted state, the balloon being attached to the distal end of the catheter and inflated via the catheter. Self-expanding stents are forcibly compacted to a small diameter and restrained at that diameter by a constraining sleeve or other means. Following delivery to a desired site for deployment, they are released from the restraint and spring open to contact the luminal surface of the body conduit. These devices are typically made from nitinol metal alloys and typically rely on the super elastic and biocompatible character of the metal. Nitinol stents that rely on the shape memory attributes of that material are also known.
The evolution of implantable stents has also included the use of a tubular covering fitted to the stent, either to the outer surface, the luminal surface or to both surfaces of the stent. These covered stents have generally come to be referred to as stent-grafts. The coverings are generally of a polymeric biocompatible material such as polyethylene terephthalate (PET) or polytetrafluoroethylene (PTFE).
It is also known that stent graft coverings may be optionally provided with perforations if desired for particular applications. Because of the open area provided by the perforations, such devices having perforated coverings may be considered to be a sort of hybrid stent and stent-graft, as are devices that include stent frames having metallic stent elements or structure and polymeric elements connecting, covering or other otherwise being attached to the stent elements. The presence of the polymeric elements reduces the otherwise open space between the adjacent metallic stent elements, either very slightly or very substantially depending on the intended application and mechanical design.
Generally, a fully covered stent-graft can be considered to have a surface area (hereinafter Amax) equal to the outer circumference of the expanded stent multiplied by the length of the stent. For a conventional, open frame stent (as opposed to a stent-graft), the surface area represented by all of the stent elements is only a small portion of the maximum surface area Amax. The actual surface area covered by the stent, meaning the area covered by all components of the stent (including flexible connecting elements and graft covering material) in their deployed state, is Astent. The porosity index, or P.I., describes the open area (the portion of the maximum surface area not covered by all components of the stent assembly) as a percentage of maximum surface area, wherein:P.I.=(1−(Astent/Amax))·times·100%.
One method of measuring the actual surface area covered by the stent (Astent), involves the use of a machine provided by Visicon Inspection Technologies, LLC (Napa, Calif.). The Visicon Finescan™ Stent Inspection System (Visicon Finescan machine model 85) uses a 6000 pixel line scan camera to generate a flat, unrolled view of a stent. The Visicon Finescan also has an updated model, the Visicon Finescan Sierra that exists and may be used alternatively to measure actual surface area. In operation, the stent is mounted on a sapphire mandrel with a fine diffuse surface. This mandrel is held under the linear array camera and rotated by the system electronics and is used to trigger the linear array camera to collect a line of image data in a precise line-by-line manner. After a complete revolution an entire image of the stent is acquired. When the entire stent has been imaged, the software differentiates between the stent with cover and the background. The total number of picture elements (pixels) is compared to the total number of pixels associated with the stent and cover to determine Astent. Basic settings on the machine used for this type of determination are (for example): light, 100%; exposure, 0.3 ms/line; gain, 5; threshold, 50; noise filter, 20; smoothing, 4.
The open area may be a continuous single space, such as the space between windings of a single helically wound stent element. Likewise the open area may be represented by the space between multiple individual annular or ring-shaped stent elements. The open area may also be represented by the total area of multiple apertures provided by either a single stent element or by multiple stent elements providing multiple apertures. If multiple apertures are provided they may be of equal or unequal sizes. The use of a perforated graft covering or of polymeric elements in addition to metallic stent elements may also reduce the open area.
Stents having a porosity index of greater than 50% are considered to be substantially open stents.
In addition to the porosity index, the size of any aperture providing the open area must be considered if it is intended to cover only a portion of a stent area for a specific stent application. For multiple apertures, often the consideration must be for the largest size of any individual aperture, particularly if the apertures are to provide for a “filtering” effect whereby they control or limit the passage of biologic materials from the luminal wall into the flow space of the body conduit.
A shortcoming of some stent devices that combine metallic stent elements with flexible polymeric connecting elements is that the non-metallic elements, (e.g., polymer webs), when constrained circumferentially or axially or when bent into a curved shape, may protrude into the luminal space of the device. This type of protrusion into the luminal space of the device may create opportunities for clinical complications such as stenosis, thrombus, altered blood hemodynamics, and associated complications.
In light of the foregoing, there is an ongoing need for endoprostheses such as stents or stent grafts that when deployed have sufficient radial force, porosity, and flexibility, while having minimal impact to blood hemodynamics and other clinical complications typically associated with interfering structures of a medical device. The embodiments described herein provide a flexible endoprosthesis (e.g. a stent or stent graft) with potentially less interference into the luminal space than currently known devices.